Not Applicable
Not Applicable
This invention relates to a preparation of anhydrides. More particularly, it relates to a transimidation-based method for the preparation of anhydrides.
Polyetherimides are high heat engineering plastics having a variety of uses. As disclosed in U.S. Pat. No. 5,229,482, one route for the synthesis of polyetherimides proceeds through a bis(4-chlorophthalimide) having the following structure (I) 
wherein X is a divalent, alkylene, cycloalkylene, or arylene moiety. The bis(4-chlorophthalimide) wherein X is a 1,3-phenyl group (II) is particularly useful. 
Bis(chlorophthalimide)s (I) and (II) are typically formed by the condensation of amines, e.g., 1,3-diaminobenzene with anhydrides, e.g., 4-chlorophthalic anhydride (III) 
4-Chlorophthalic anhydride is an expensive starting material which presently must be custom synthesized. Current routes for the synthesis of 4-chlorophthalic anhydride lead to mixtures of isomers, which are difficult to separate, or are prohibitively costly. For example, the Diels-Alder condensation of maleic anhydride with 2-chloro-1,3-butadiene to yield 4-chlorotetrahydrophthalic anhydride, followed by aromatization in the presence bromine requires the subsequent recovery of HBr. Attempted thermal aromatization of 4-chloro-tetrahydrophthalic anhydride results in low yields of 4-chlorophthalic anhydride and tar formation. This and other routes are described in U.S. Pat. Nos. 5,059,697; 5,003,088; 5,322,954; 4,978,760 and 5,233,054.
A new method for the synthesis of phthalic anhydrides (IV) 
wherein Rxe2x80x2 is a halogen, an aromatic or aliphatic group comprising 1 to about 18 carbons, a hydrogen or a nitro group is the transimidation between the corresponding N-alkyl phthalimide (V) 
wherein Rxe2x80x2 in (IV) and Rxe2x80x2 in (V) are identical, and further wherein R is an alkyl group having from 1 to about 18 carbons, and a tetrahydrophthalic an hydride (VI) 
The by-product of the transimidation is an N-alkyl tetrahydrophthalimide (VII) 
In an advantageous feature of this method, where Rxe2x80x2 in (V), (VI) and (VII) are identical, N-alkyl tetrahydrophthalimide (VII) may be converted by aromatization to produce the corresponding N-alkyl phthalimide (V).
A convenient route for the formation of phthalic anhydrides (IV) is via transimidation between the corresponding N-alkyl phthalimide (V) and a tetrahydrophthalic anhydride (VI). A by-product of this reaction is an N-alkyl tetrahydrophthalimide (VII). This by-product is preferably converted to yield the corresponding N-alkyl phthalimide (V).
The product phthalic anhydrides (IV) have the structure 
wherein Rxe2x80x2 is a halogen, an aromatic or aliphatic group comprising 1 to about 18 carbons, a hydrogen or a nitro group.
The starting N-alkyl phthalimides (V) have the structure 
wherein Rxe2x80x2 is as described above, and R is a straight chain or branched alkyl group having from 1 to about 18 carbons, for example methyl, ethyl, propyl, and the like. N-alkyl phthalimides (V) may be obtained from the corresponding N-alkyl tetrahydrophthalimide by aromatization. Aromatization may be achieved by any method known in the art, such as those taught by U.S. Pat. Nos. 5,233,054; 5,003,088; 5,059,697 and 4,978,760. Aromatization may also be achieved by passing the N-alkyl tetrahydrophthalimide over a transition metal catalyst such as V2 O5 at a temperature in the range of about 250xc2x0 C. to about 270xc2x0 C. Alternately, N-alkyl phthalimides (V) may be obtained by heating a tetrahydrophthalic anhydride with the desired alkyl amine or aryl amine at a temperature of about 50xc2x0 C. to about 250xc2x0 C. for up to about 5 hours. The resulting product, N-alkyl phthalimide may be isolated by any method known in the art such as distillation or column chromatography.
Tetrahydrophthalic anhydrides (VI) 
wherein Rxe2x80x2 is as defined above may be obtained by the Diels-Alder condensation of dienophile maleic anhydride with a diene substituted by Rxe2x80x2. Conditions for this reaction are known in the chemical literature. Suitable Rxe2x80x2 substitutions include, but are not limited to, halogen, aromatic or aliphatic groups comprising 1 to about 18 carbons, hydrogen or nitro group. A preferred diene is 2-chloro-1,3- butadiene (chloroprene).
An advantageous feature of this method is that where Rxe2x80x2 are identical in both (V) and (VI), the by-product of the transimidation, N-alkyl tetrahydrophthalimide (VII) 
can be converted by aromatization to produce N-alkyl phthalimide (V) as discussed above. Due to this advantageous feature it is contemplated that the synthesis of 4-chlorophthalic anhydrides may be practiced in a batchwise or continuous fashion.
As shown in Scheme I this method is particularly suitable for the formation of 4-chlorophthalic anhydride (III), which is an important intermediate in the synthesis of polyetherimides. 
Accordingly, 4-chlorophthalic anhydride (III) is produced by transimidation of N-methyl-4-chlorophthalimide (VIII) with 4-chlorotetrahydrophthalic anhydride (IX). The by-product of the transimidation, N-methyl-4-chlorotetrahydrophthalimide (X) is preferably converted by aromatization to N-methyl-4-chlorophthalimide (VIII).
All patents cited herein are incorporated by reference.